1. Field of the Invention
The present invention relates to liquid crystal display (LCD) devices and more particularly, to LCD devices using a plastic substrate.
2. Discussion of the Related Art
Flat panel display (FPD) devices having small size, lightweight, and low power consumption have been a subject of recent research in the coming of the information age. Among many kinds of FPD devices, LCD devices are widely developed and used for notebook and desktop personal computers (PC's) because of their excellent characteristics of resolution, color display and display quality.
Generally, an LCD device includes an upper substrate and a lower substrate facing each other with liquid crystal molecules interposed therebetween. Each substrate has an electrode on an inner surface thereof. An electric field is generated by applying a voltage to the electrodes, thereby driving the liquid crystal molecules to display images depending on light transmittance.
FIG. 1 is a schematic cross-sectional view of a conventional liquid crystal display device. As shown in FIG. 1, a first substrate 10 and a second substrate 20 are spaced apart from each other. The first substrate 10 and the second substrate 20 are also known as a lower substrate and an upper substrate, respectively. A gate electrode 11 is formed on an inner surface of the first substrate 10 and a gate insulating layer 12 is formed on the gate electrode 11. An active layer 13 is formed on the gate insulating layer 12 over the gate electrode 11, and an ohmic contact layer 14 is formed on the active layer 13. Source and drain electrodes 15a and 15b are formed on the ohmic contact layer 14 and constitute a thin film transistor (TFT) “T” along with the gate electrode 11. A passivation layer 16 is formed on the TFT “T.” The passivation layer 16 has a contact hole 16c exposing the drain electrode 15b. A pixel electrode 17 of a transparent conductive material is formed on the passivation layer 16 and connected to the drain electrode 15b through the contact hole 16c. 
A black matrix 21 corresponding to the TFT “T” is formed on an inner surface of the second substrate 20. A color filter layer 22a and 22b alternately having colors of red (R), green (G) and blue (B) is formed on the black matrix 21. A common electrode 23 made of a transparent conductive material is formed on the color filter layer 22a and 22b. Here, one color of the color filter layer 22a and 22b corresponds to one pixel electrode 17.
A liquid crystal layer 30 is interposed between the pixel electrodes 17 and the common electrode 23. When a voltage is applied to the pixel electrode 17 and the common electrode 23, an arrangement state of liquid crystal molecules of the liquid crystal layer 30 is changed by an electric field generated between the pixel electrode 17 and the common electrode 23. An orientation film (not shown) on each of the pixel electrode 17 and the common electrode 23 determines an initial alignment state of the liquid crystal molecules.
The conventional LCD device is fabricated through a process of forming the first substrate having the TFT and the pixel electrode, a process of forming the second substrate having the color filter layer and the common electrode, and a process of forming a liquid crystal cell. The process of forming the liquid crystal cell includes aligning the first and second substrates, injecting a liquid crystal material between the first and second substrates, sealing the LCD device, and attaching a polarizing plate to the LCD device. Here, the polarizing plate disposed on each outer surface of the first and second substrates 10 and 20 linearly polarizes natural light by transmitting light parallel to an optic axis of the polarizing plate. Since the liquid crystal cell does not emit light, an additional light source is necessary. Therefore, a backlight is disposed over the polarizing plate on the outer surface of the first substrate 10. Hence, the conventional LCD device displays images by using light emitted form the backlight and adjusting an amount of the light according to the alignment state of the liquid crystal molecule. Here, the pixel electrode 17 and the common electrode 23 generating the electric field are made of a transparent material. Moreover, the first and second substrates 10 and 20 are also made of a transparent material.
Generally, a glass substrate is used as the first and second substrates 10 and 20 in conventional LCD devices. However, since the glass substrate is heavy and fragile, it adds to the weight and cost of the LCD device and deteriorates the durability of the LCD device.
FIGS. 2A and 2B are schematic cross-sectional views showing loading states of a glass substrate and a plastic substrate, respectively. As shown in FIGS. 2A and 2B, a glass substrate 52 is closely loaded on a stage 51 of an apparatus due to its flatness and thus, the subsequent process is smoothly performed. On the other hand, a plastic substrate 62 is flexible and is not closely loaded on a stage 61.